Avoid creating instances of nested procedurals that contain a single object

[sebastienblor]

When we render an instance of a non-renderable primitive, we have to create a nested procedural. However, quite frequently, we end up creating such a procedural for a procedural that will create a single node. In that case we should manage to use this leaf node as the actual prototype for our instance

[autodesk-oss-arnold-bot[bot]]

Issue synced internally to ARNOLD-14630